Non-nil means print recursive structures using #N= and #N# syntax.
If nil, printing proceeds recursively and may lead to
`max-lisp-eval-depth' being exceeded or an error may occur:
"Apparently circular structure being printed." Also see
`print-length' and `print-level'.
If non-nil, shared substructures anywhere in the structure are printed
with `#N=' before the first occurrence (in the order of the print
representation) and `#N#' in place of each subsequent occurrence,
where N is a positive decimal integer.